As a cover material of golf balls, ionomer resin has been widely used (Japanese Kokai Publication 49727/1974).
In particular, for two-piece golf balls which contain solid cores, almost all balls use the ionomer resin as cover material. It is because the ionomer resin has excellent durability, anti-cutting property, preferred rebound property and easy processing property and it is cheaper than other cover materials usable for golf balls.
However, the ionomer resin is inferior to a Balata cover (a trans-polyisoprene cover) which is used for the cover material of thread-wound golf balls, in regard to hit feeling and controllability (easiness in giving spin to the ball), because of high hardness and high stiffness. Many trials have been made to improve the hit feeling and controllability of the golf ball by making the ionomer resin flexible.
Among them, Japanese Kokai Publication 308577/1989 discloses a flexible ionomer being a sodium salt or zinc salt of a terpolymer which is prepared from an olefine having 2 to 8 carbon atoms, an unsaturated monocarboxylic acid having 3 to 8 carbon atoms and an acrylate monomers having 2 to 22 carbon atoms. The flexible ionomer resin do not have sufficient properties for golf balls.